1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device including a step of forming a thin film on a substrate, and relates to a substrate processing apparatus.
2. Description of the Related Art
A flash memory includes an electronic storage region (floating gate) surrounded by an insulating film, and the operating principle is that information is written through exchange of electrons via a thin tunnel oxide film, and simultaneously electrons are retained for a long time by utilizing insulating property of this thin oxide film to keep memory. The information stored in the flash memory must be retained for as long as 10 years even if no external operation is performed, and there is a severe requirement for the insulating film surrounding a charge storage region called the floating gate. A stacked structure of an oxide film (SiO2)/a nitride film (Si3N4)/an oxide film (SiO2), which is generally called oxide-nitride-oxide (ONO), is used in an inter-layer insulating film provided between the floating gate and a control gate for controlling operation of a memory cell, and high leak current characteristics are expected.
Conventionally, the SiO2 insulating film in the ONO stacked structure is formed in a high temperature region of about 800° C. by a CVD method using the gas such as SiH2Cl2 gas and N2O gas. With further microfabrication of a device, capacity drop of the nitride film in the ONO stacked film occurs. Accordingly, from a viewpoint of ensuring capacity, adoption of a high dielectric constant film replacing a nitride film layer is being investigated.
In order to suppress crystallization of the high dielectric constant film, the SiO2 insulating film formed on a high dielectric constant film must be formed at a temperature lower than the temperature for forming the high dielectric constant film.
When an SiO2 insulating film is formed, along with a lower temperature for forming the film, atoms other than silicon (Si) and oxygen (O) contained in a source used for forming the film are likely to remain in the film as impurities. Therefore, when the SiO2 insulating film is formed at a low temperature by using organic source gas, there is a problem that carbon (C), hydrogen (H), nitrogen (N), etc, contained in organic source gas molecules remain in the SiO2 insulating film as impurities. In addition, even when an inorganic source gas is used, there is a problem that hydrogen (H), chlorine (Cl), etc, contained in the source remain in the film as impurities.
Since these impurities significantly deteriorate film quality of the formed insulating film, there is a necessity for providing a technique of forming an insulating film of good quality with low concentration of impurities in the film at a low temperature.
Therefore, an object of the present invention is to provide a method for manufacturing a semiconductor device and a substrate processing apparatus, capable of solving the above-described problem, and forming an insulating film with extremely low concentration of the impurities in the film such as carbon, hydrogen, nitrogen, chlorine, etc., at a low temperature.